Alkaline sorbent injection for mercury control

ABSTRACT

A mercury removal system for removing mercury from combustion flue gases is provided in which alkaline sorbents at generally extremely low stoichiometric molar ratios of alkaline earth or an alkali metal to sulfur of less than 1.0 are injected into a power plant system at one or more locations to remove at least between about 40% and 60% of the mercury content from combustion flue gases. Small amounts of alkaline sorbents are injected into the flue gas stream at a relatively low rate. A particulate filter is used to remove mercury-containing particles downstream of each injection point used in the power plant system.

[0001] The subject matter of the present invention was developed under aresearch contract with the U.S. Department of Energy (DOE), Contract No.DE-FC22-94PC94251, and under a grant agreement with the Ohio CoalDevelopment Office (OCDO), Grant Agreement No. CDO/D-922-13. Thegovernments of the United States and Ohio have certain rights in theinvention.

FIELD AND BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to the field ofcombustion and flue gas cleanup methods and apparatus and, inparticular, to a new and useful apparatus and method for removingmercury from flue gases generated by combustion, through the use of analkaline sorbent.

[0003] In recent years, the U.S. Department of Energy (DOE) and the U.S.Environmental Protection Agency (EPA) have supported research to measureand control the emissions of Hazardous Air Pollutants (HAPS) fromcoal-fired utility boilers and waste to energy plants. The initialresults of several research projects showed that the emissions of heavymetals and volatile organic carbons (VOCs) are very low, except formercury (Hg). Unlike most of the other metals, mercury remains in thevapor phase at relatively low temperatures and does not condense ontofly ash particles. Therefore, it cannot be collected and disposed ofalong with fly ash like the other metals. To complicate matters, mercurycan exist in its oxidized (Hg⁺⁺) or elemental (Hg⁰) form and each isaffected differently by subsequent downstream pollution controlequipment.

[0004] Most of the recent efforts to capture and remove mercury from theflue gas produced by coal-fired units have concentrated on gas-phasereactions with introduced reagents such as activated carbon.

[0005] The subject of mercury emissions by the utility and waste toenergy industries is a new area being investigated by both the DOE andEPA.

[0006] Approximately 75% of existing coal-fired power plants are notequipped with wet flue gas desulfurization (WFGD) systems. These systemsmost often control particulate emissions with electrostaticprecipitators (ESP's) and baghouses. With possible mercury emissionsregulation for the electric power industry pending, it is imperative tohave cost-effective mercury capture technologies available for thosepower plants lacking WFGD systems.

[0007] It is known to inject limestone in dry powder form into the fluegases in the upper furnace cavity of a boiler for the purpose ofcapturing SO₂ from the flue gases. A discussion of systems using thisprocess can be found in U.S. Pat. Nos. 5,795,548 and 5,814,288 to Maddenet al. These systems or processes are also referred to as EnhancedLimestone Injection Dry Scrubbing processes/systems, or E-LIDS systems™,a trademark of The Babcock & Wilcox Company. Please refer to FIG. 1.

[0008] For the E-LIDS™ processes or systems, a particulate collectiondevice is located downstream of the air heater to remove particulatematter from the flue gases exiting the boiler. Any one of several knowntypes of particulate separation techniques may be employed for thispurpose, including inertial impaction separators, fabric filters(baghouses) and ESP's. Flue gases exiting from the particulate collectorthen pass through a dry scrubber where they are contacted by a slurrycontaining calcium hydroxide. Calcium is introduced in stoichiometricmolar ratios of calcium to sulfur much greater than 1.0 and usuallyabout 2.0 mole/mole. The high molar ratios are necessary to achieve goodreactions between the calcium and sulfur present in the flue gases.

[0009] Additional SO₂ removal can take place in a dry scrubber locateddownstream of the particulate control device, followed by a finalparticulate collector in which coal flyash, spent sorbent and unreactedsorbent particles are collected. A baghouse is preferred as the finalparticulate control device because of the additional SO₂ removal ityields as the flue gases pass through the filter cake on the filterbags. Thus, the E-LIDS™ process or system combines sorbent injection,dry scrubbing and fabric filtration.

SUMMARY OF THE INVENTION

[0010] It has been discovered that the E-LIDS™ process also has theeffect of removing 95% of the total amount of mercury present in thefurnace system. Surprisingly, it was discovered that 82% of the mercuryremoval occurred using the sorbent injection and first particulatecollector alone.

[0011] It is an object of the present invention to provide a costefficient solution for reducing mercury emissions in flue gases that iseasily retrofit into existing power plant systems.

[0012] Accordingly, one aspect of the present invention is drawn to amercury removal system for removing mercury from a flue gas generated inutility and waste to energy combustion systems having a boiler and astack, comprising: particulate removal means for separating and removingparticulate matter containing mercury from the flue gas, the particulateremoval means located between the boiler and the stack; and sorbentinjection means for providing an alkaline sorbent in one of powdered andslurried form to at least one location upstream of the particulateremoval means in the power plant, the alkaline sorbent being provided ina stoichiometric molar ratio of calcium to sulfur in a range of about0.001 mole of an alkaline earth or an alkali metal/mole sulfur and 1.0mole of an alkaline earth or an alkali metal/mole sulfur. The alkalinesorbents are injected into a power plant system at one or more locationsand at stoichiometric molar ratios of and alkaline earth or an alkalimetal to sulfur of less than 1.0 to remove at least between about 40%and 60% of the mercury content from power plant emissions. Small amountsof alkaline sorbents are thus injected into the flue gas stream at arelatively low rate. A particulate filter is used to removemercury-containing particles downstream of each injection point used inthe power plant system.

[0013] Under certain circumstances, it may be desirable to use acombination of both an alkaline earth sorbent and an alkali metalsorbent to accomplish mercury removal according to the presentinvention.

[0014] The various features of novelty which characterize the inventionare pointed out with particularity in the claims annexed to and forminga part of this disclosure. For a better understanding of the invention,its operating advantages and specific benefits attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the drawings:

[0016]FIG. 1 is a schematic diagram of a power plant installationincorporating an E-LIDS™ system according to the prior art;

[0017]FIG. 2 is a schematic diagram of a power plant installationincorporating the alkaline sorbent injection system for mercury controlaccording to the present invention; and

[0018]FIG. 3 is a bar graph showing amounts of mercury captured, asmeasured by testing equipment, following flue gas treatments accordingto the present invention as compared to unfiltered flue gases.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Referring now to the drawings, wherein like reference numeralsdesignate the same or functionally similar elements throughout theseveral drawings, FIG. 2 shows a power plant installation or system 10having an alkaline sorbent preparation means 12 for preparing analkaline sorbent 14 which is conveyed via lines 15, 16, 18, 20, and/or22 to various locations of system 10. The power plant system 10 includesa boiler 24 having a lower furnace region 26, an upper furnace region28, a convection pass 30, an air heater 32, emissions control devices34, and a stack 36. Fuel 38, typically coal, and air 40 for combustionare provided into the boiler 24 to generate heat via combustion.

[0020] In the system 10 shown, hot flue gases 42, containingcontaminants such as mercury, are generated in the boiler 24 furnace andrise through upper furnace region 28. The flue gases 42 then passthrough the convection pass section 30 of the boiler 24 before enteringair heater 32. After air heater 32, cooled flue gases 44 may be treatedby one or more emissions control devices, generally designated 34.Desulfurized and cleaned (of particulate) flue gases 46 exit fromdevices 34 and are conveyed to stack 36 for release into the atmosphere.

[0021] Emissions control devices 34 may include baghouses, electrostaticprecipitators, WFGD systems, wet scrubbers, dry scrubbers, selectivecatalytic reduction (SCR), selective non-catalytic reduction (SNCR), andimpact-type particle separators. However, as noted above, many existingpower plant systems 10 do not have WFGD systems, and use only an ESP orbaghouse to control emissions. In systems where a WFGD system isinstalled, typically an ESP will be placed upstream to removeparticulate matter prior to the flue gas entering the WFGD system.

[0022] As illustrated in FIG. 2, the alkaline sorbent may be deliveredinto the flue gases 42, 44, 46 at one or more locations of the upperfurnace region 28, the convection pass 30, at the emissions controldevices 34, prior to exiting the system 10 through the stack 36, or inwith the fuel 38. Suitable alkaline sorbents 14 include sorbentscontaining elements selected from the alkali metals (Group 1a of theperiodic table) such as sodium, or the alkaline earths (Group 2a of theperiodic table) which includes calcium-based sorbents such as limestoneand lime. The alkaline sorbent 14 may be in slurry or powdered form, andthe means 15, 16, 18, 20, and 22 would of course be designed to conveythe sorbent 14 to the desired locations in whatever form it is provided,whether in slurry or powder form.

[0023] Alkaline sorbent injection for mercury control includes theinjection of any alkaline sorbent 14 into a flue gas 42, 44, 46 streamanywhere from the boiler 24 to the exit of the stack 36 at very smallamounts (Ca/S stoichiometries less than 1.0 mole/mole) for the purposeof mercury capture. The sorbent 14 can be injected into the flue gas 42,44, 46 stream dry or as slurry. The injected sorbent 14 absorbs oradsorbs the mercury into the particulate phase allowing for thecollection of the mercury with the solids in the flue gas in downstreamemissions control devices 34. The temperatures for injection of thesorbent range from those typical at the coal input to the boiler (3000°F.) and in the upper portion 28 of a furnace (2300° F.) to very lowtemperatures such as at the outlet of a wet scrubber (150° F.). Eachfacility's flue gas constituents and equipment will dictate the type orsorbent and where (what temperature) to inject. In FIG. 2, the solidarrow (line 16) from the sorbent preparation system 12 is a recentlytested application of the present invention that is known to work. Thisis injection into the upper furnace region 28. The dashed arrows fromthe sorbent preparation system 12 are other injection points for sorbent14 injection for mercury capture according to the principles of theinvention that are expected to work; however, these applications are yetto be tested (examples include introduction with the coal feed 38, inthe convection pass 30, anywhere in the flue gas desulfurization andparticulate control device section 34 and before the stack 36).

[0024] Recent testing performed as part of the above-identified contractwith the DOE has surprisingly demonstrated that the injection of evenvery small amounts of limestone sorbent 14 (i.e., calciumstoichiometries between 0.04 and 0.35 mole Ca/mole S) via line 16 intothe upper furnace region 28 of the boiler 24 can achieve modest mercuryremoval from the flue gases 42. This is a new and unique application foralkaline sorbent injection. Previously, such injection of alkalinesorbent was used for the removal of SO₂ from flue gases, and it was alsoinjected at much higher flow rates (i.e., calcium stoichiometriesbetween 1.4 and 2.0 mole Ca/mole S). Table 1 below summarizes thesorbent injection operating conditions for the one specific applicationtested, while FIG. 3 graphically illustrates the test data obtained whenalkaline sorbent (limestone) 14 was injected into the upper furnaceregion 28 of boiler 24. TABLE 1 Sorbent Injection Operating Conditionsfor Specific Test Application Sorbent Limestone Limestone # of sorbent/#of flue gas 0.002 0.00025 Ca/S ratio, mole/mole 0.35  0.04   Sorbent/HgWt. ratio 125,000:1 16,000:1 Injection Temp, ° F. 2200 2200 ESP Temp, °F.  350  350 Total Hg Removal 56% 45%

[0025] As illustrated in FIG. 3, bar 200 represents the uncontrolledemissions from a test power plant system 10 in which about 70% of themercury is oxidized mercury, another approximately 20% is particulatephase mercury, and the remainder is elemental mercury. About 23 μg/dscm(23 micrograms/dry standard cubic meters) total of mercury was observedin the uncontrolled emissions.

[0026] Bar 250 shows the effect of using only an electrostaticprecipitator on mercury removal. Approximately 18% of the total mercurypresent in the uncontrolled emissions (bar 200) is removed.

[0027] A comparison of bar 200 with bars 300 and 350, representingemissions when an alkaline sorbent 14 has been injected into the powerplant system 10, clearly shows the beneficial reduction in mercuryemitted into the atmosphere by the furnace combustion process.

[0028] Bar 300 shows the total amount of mercury observed afterinjecting limestone in a stoichiometric molar ratio of 0.35 calcium tosulfur, or at a rate of 0.002 lbs. of limestone per pound of flue gas,into the upper furnace region 28. The total mercury emissions arereduced substantially; 56% of the mercury is removed from theuncontrolled emissions by the alkaline sorbent 14 injection.

[0029] In a second test, the results of which are shown by bar 350,limestone was injected into the upper furnace region 28 at astoichiometric molar ratio of about 0.04 calcium to sulfur, or at a rateof 0.00025 pounds of limestone per pound of flue gas. The lower molarratio yields less mercury control, with about 45% of the total mercuryis removed from the uncontrolled emissions. Returning to FIG. 2, theinjection system used to provide the alkaline sorbent 14 to each of thedifferent locations in the power plant system 10 may be of any knowntype for delivering powdered or slurried substances, such as pumps or anair transport system. One advantage of the invention is the alkalinesorbent 14 can be provided from a retro-fit component having arelatively small footprint relative to a full WFGD system for thosepower plants lacking a WFGD. The cost to install such an injectionsystem is considerably lower than that for a WFGD system.

[0030] Since relatively small amounts of alkaline sorbent 14 areinjected into the power plant system 10, the cost to provide thealkaline sorbent 14 is relatively inexpensive. Smaller storage silos maybe used as well, contributing to a small footprint for an injectionretrofit.

[0031] The alkaline sorbent injection of the invention also providesadditional control over sulfur oxides emissions for plants beingretrofit and which lack WFGD systems. The alkaline sorbent 14 injectedinto the power plant system 10 has the added effect of removing, andthereby reducing amounts of SO₃, HCl and other acid gases from the fluegases while also reducing the mercury content.

[0032] Lower SO₃ levels provide the benefit of reduced acid dew point,allowing further heat removal from the flue gases, as the temperaturecan be lowered further without generating caustic and corrosivecondensate. In turn, lower heat levels for the flue gases at theparticulate collection device increases the potential amount of mercurythat can be removed, as well as increasing boiler efficiency.

[0033] Under certain circumstances, it may be desirable to use acombination of both an alkaline earth sorbent and an alkali metalsorbent to accomplish mercury removal according to the presentinvention.

[0034] Finally, fly ash carbon content can be diluted due to theinjection of the alkaline sorbent 14. The amount of unburned carbonfound in the fly ash at power plants often dictates the availability ofthe ash for utilization methods. Diluting the fly ash makes the unburnedcarbon percentages lower, and thus, the ash will be more desirable forcommercial sale. Increased alkalinity of the ash can increase the valueof the ash for several applications such as in the agricultural andconcrete industries.

[0035] While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

We claim:
 1. A mercury removal system for removing mercury from a fluegas generated in a combustion system having a boiler and a stack,comprising: particulate removal means for separating and removingparticulate matter containing mercury from the flue gas, the particulateremoval means located between the boiler and the stack; and sorbentinjection means for providing an alkaline sorbent in one of powdered andslurried form to at least one location upstream of the stack, thealkaline sorbent containing at least one element selected from thealkaline earth elements, the sorbent further being provided in astoichiometric molar ratio of alkaline earth to sulfur in a range ofabout 0.001 mole alkaline earth/mole sulfur to about 1.0 mole alkalineearth/mole sulfur.
 2. The mercury removal system according to claim 1,wherein the stoichiometric molar ratio is in a range of about 0.001 molealkaline earth/mole sulfur to about 0.5 mole alkaline earth/mole sulfur.3. The mercury removal system according to claim 2, wherein the alkalinesorbent comprises one of limestone, lime, a calcium-based sorbent, and acombination thereof.
 4. The mercury removal system according to claim 3,wherein the alkaline sorbent comprises limestone.
 5. The mercury removalsystem according to claim 1, wherein the stoichiometric molar ratio isabout 0.35 mole alkaline earth/mole sulfur.
 6. The mercury removalsystem according to claim 5, wherein the alkaline sorbent comprises oneof limestone, lime, a calcium-based sorbent, and a combination thereof.7. The mercury removal system according to claim 1, comprising means forproviding the alkaline sorbent to the boiler.
 8. The mercury removalsystem according to claim 7, wherein the alkaline sorbent is provided toat least one of an upper furnace region and a convection pass of theboiler.
 9. The mercury removal system according to claim 7, wherein thealkaline sorbent is one of limestone, lime, a calcium-based sorbent, anda combination thereof.
 10. The mercury removal system according to claim7, wherein the particulate removal means comprises an electrostaticprecipitator.
 11. The mercury removal system according to claim 1,wherein the particulate removal means comprises an electrostaticprecipitator.
 12. The mercury removal system according to claim 11,wherein the stoichiometric molar ratio is in a range of about 0.001 molealkaline earth/mole sulfur to about 0.5 mole alkaline earth/mole sulfur.13. The mercury removal system according to claim 11, comprising meansfor providing the alkaline sorbent to the boiler.
 14. The mercuryremoval system according to claim 13, wherein the alkaline sorbent isprovided to at least one of an upper furnace region and a convectionpass of the boiler.
 15. The mercury removal system according to claim14, comprising means for providing the alkaline sorbent to at least oneof upstream adjacent the electrostatic precipitator and upstreamadjacent the stack, the stack further comprising separator means forseparating and removing particulate containing mercury from the fluegas.
 16. The mercury removal system according to claim 11, comprisingmeans for providing the alkaline sorbent to at least one of upstreamadjacent the electrostatic precipitator and upstream adjacent the stack,the stack further comprising separator means for separating and removingparticulate containing mercury from the flue gas.
 17. A mercury removalsystem for removing mercury from a flue gas generated in a combustionsystem having a boiler and a stack, comprising: particulate removalmeans for separating and removing particulate matter containing mercuryfrom the flue gas, the particulate removal means located between theboiler and the stack; and sorbent injection means for providing analkaline sorbent in one of powdered and slurried form to at least onelocation upstream of the stack, the alkaline sorbent containing at leastone element selected from the alkali metals, the sorbent further beingprovided in a stoichiometric molar ratio of alkali metal to sulfur in arange of about 0.001 mole alkali metal/mole sulfur to about 1.0 molealkali metal/mole sulfur.
 18. The mercury removal system according toclaim 17, wherein the stoichiometric molar ratio is in a range of about0.001 mole alkali metal/mole sulfur to about 0.5 mole alkali metal/molesulfur.
 19. The mercury removal system according to claim 18, whereinthe alkaline sorbent comprises a sodium-based sorbent.
 20. The mercuryremoval system according to claim 17, wherein the stoichiometric molarratio is about 0.35 mole alkali metal/mole sulfur.
 21. The mercuryremoval system according to claim 20, wherein the alkaline sorbentcomprises a sodium-based sorbent.
 22. The mercury removal systemaccording to claim 17 comprising means for providing the alkalinesorbent to the boiler.
 23. The mercury removal system according to claim22, wherein the alkaline sorbent is provided to at least one of an upperfurnace region and a convection pass of the boiler.
 24. The mercuryremoval system according to claim 22, wherein the alkaline sorbent is asodium-based sorbent.
 25. The mercury removal system according to claim22, wherein the particulate removal means comprises an electrostaticprecipitator.
 26. The mercury removal system according to claim 17,wherein the particulate removal means comprises an electrostaticprecipitator.
 27. The mercury removal system according to claim 26,wherein the stoichiometric molar ratio is in a range of about 0.001 molealkali metal/mole sulfur to about 0.5 mole alkali metal/mole sulfur. 28.The mercury removal system according to claim 26, comprising means forproviding the alkaline sorbent to the boiler.
 29. The mercury removalsystem according to claim 28, wherein the alkaline sorbent is providedto at least one of an upper furnace region and a convection pass of theboiler.
 30. The mercury removal system according to claim 29, comprisingmeans for providing the alkaline sorbent to at least one of upstreamadjacent the electrostatic precipitator and upstream adjacent the stack,the stack further comprising separator means for separating and removingparticulate containing mercury from the flue gas.
 31. The mercuryremoval system according to claim 26, comprising means for providing thealkaline sorbent to at least one of upstream adjacent the electrostaticprecipitator and upstream adjacent the stack, the stack furthercomprising separator means for separating and removing particulatecontaining mercury from the flue gas.
 32. A mercury removal system forremoving mercury from a flue gas generated in a combustion system havinga boiler and a stack, comprising: particulate removal means forseparating and removing particulate matter containing mercury from theflue gas, the particulate removal means located between the boiler andthe stack; and sorbent injection means for providing an alkaline sorbentin one of powdered and slurried form to at least one location upstreamof the stack, the alkaline sorbent containing a combination of at leastone element selected from the alkaline earth elements and at least oneelement selected from the alkali metals, the sorbent further beingprovided in a stoichiometric molar ratio of alkaline earth and alkalimetal to sulfur in a range of about 0.001 mole alkaline earth and alkalimetal/mole sulfur to about 1.0 mole alkaline earth/mole sulfur.